Hello Piston Slappers, this S.O.S. is for anyone GM-savvy enough to tango with a 4.1L Cadillac. And live to diagnose another day: are you up for the challenge?

I acquired a 1986 Fleetwood 75 Formal Limousine (1 of 1000), which made me reconsider my stance on wrong-wheel drive GM products of the mid 1980s This was a $0.99 eBay purchase from a frustrated shipping company in the Houston Ship Channel who lost their overseas buyer. It didn’t run, until we installed a $20 ignition module. Which brought the less-than-a-buck Caddy back to life. Almost. So what’s the problem?

Tech Overload Warning! After fresh oil, coolant, belt, filters, vacuum lines, T-stat and a successful compression test, the trouble-prone HT4100 (at 84,000 miles) cannot idle below 1200 rpm, is hard to start, stalls going into gear and generates an E30 (ISC RPM Out Of Range) code. The ISC motor works and seems to be adjusted correctly. The TPS sensor is fine, according to the (ingenious) self-test on the dashboard. But these items needed replacement: coil, radiator, temperature sensor and alternator. While currently under $500 of capital invested, I’m now officially weary of throwing parts at this
problem.

So I am leaning toward addressing the Internet’s two biggest beefs with this Caddy: intake manifold gaskets (is there a horrendous vacuum leak?) or a worn distributor gear. There is some slack in the rotor, but those intake gaskets might be on their way out. Any guesses on resolving my problem? How offbeat is my latest diagnosis?

(Don’t waste bandwidth telling me to scrap the car: this Fleetwood has Farago-worthy levels of Cadillac brand equity in its copious hindquarters. Even the staffers at the 24 Hours of LeMons are in love. Chat with Autoblog’s Jonny Lieberman and Jalopnik’s Murilee Martin if you don’t believe me. More on that later.)

What model years offered this engine? I remember my 82 Sedan deVille having 4100 badges on it. Is that the same engine? If so, mine at 160,000+ miles still ran strong and always started, even in the coldest winter weather. Perhaps it is different as the later 80s brought on much smaller cars and perhaps they tweaked the engines (making them unreliable).

Just because the car’s self test reports that the TPS is good doesn’t mean it is so. The self diagnostic is telling you that the TPS is not shorted, it can’t tell you if it’s “scratchy”, or slightly out of range.

I owned a Caddy with the 4.9(a larger displacement version of the 4.1) and I had the same problem — idled at 1,200, ISC RPM Out Of Range code, and “good” TPS.

I went the same diagnostic route you are now doing. I tested for manifold vacuum leaks by spraying carb cleaner around the mating surfaces — if there’s a leak, the rpm will change when it sucks in the carb cleaner. I doubt the distributor gear is worn at only 84K miles — assuming the car had regular oil changes.

After replacing numerous ignition parts, ISC motor, fuel filter, fuel pressure regulator, etc, it turned out to be the “good” TPS. After replacing the TPS, and doing an idle relearn, it ran as smooth as could be.

BTW, here is a Caddy resource you may find helpful:

http://www.cadillacforums.com/forums/

ETA: DON’T FORGET TO CLEAN THE THROTTLE PLATES BEFORE YOU DO ANYTHING ELSE!

1. Coolant leaks into the oil around the bottom of the cylinder liners.

2. Coolant leaks into teh oil right through the aluminum block, due to porosity.

3. Head gaskets leak coolant into teh oil. Iron heads on an aluminum block, bad idea. Copied from the Vega.

4. Aluminum block distorts under load, weaing bearings badly.

5. Iron heads, aluminum block and intake manifold conspire to warp the intake manifold, causing leaks of coolant and air into intake manifold.

6. Exhaust valves break, causing catastrophic engine failure.

#4 will cause problems with the enigne control system, causing odd symptoms.

But, cheer up. This engine was #1 in J.D. Powers survey for “Initial Engine Quality at Delivery”. In other words, it felt pretty good when you left the dealership, new.

The following HT 4500 and 4.9 engines improved, at least a little. They will also fit where an HT4100 fit, so it is a relatively easy swap. I would recomend this path rather than wasting time on an HT4100.

Hey, easy on the POS Eta engines from BMW. I drove an 87 528e for many years, seeing 25 mpg average overall and up to 35 mpg on the highway.

And yet, on the track I embarrased many newer BMW’s, including some less than expertly driven E36 M3’s and the like. I did have a chip worth a few more HP, and stiffer shocks, but the car handled just like any BMW. Truly a momentum car, I would have to be careful not to trail someone to close into a turn, or the engine revs would fall so low as I ran up behind them that it would take forever to climb back out!

I’m thinking possibly the Idle Air Control valve might be having a problem. Check out this comment with a link to a Cadillac forum (you have to register to follow the link, I didn’t): http://townhall-talk.edmunds.com/direct/view/.f18a31a/35#MSG35

FreedMike : My point: Caddy was NOT alone in making POS cars in the mid-’80s.

Um, sure. Too bad Caddy made the shittiest of them all, by an unbelievably wide margin. (FWIW, this thing woulda been crushed months ago if it wasn’t a gen-u-ine Fleetwood 75.)

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jwltch : What model years offered this engine? I remember my 82 Sedan deVille having 4100 badges on it. Is that the same engine? If so, mine at 160,000+ miles still ran strong and always started, even in the coldest winter weather.

Yes, they are the same, but you had chrome valve covers and mine has numerous internal upgrades (timing chain for one) that came under its black finned covers. So your positive opinions are quite reassuring. Thanks.

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skor : I went the same diagnostic route you are now doing. I tested for manifold vacuum leaks by spraying carb cleaner around the mating surfaces — if there’s a leak, the rpm will change when it sucks in the carb cleaner. I doubt the distributor gear is worn at only 84K miles — assuming the car had regular oil changes.

I will do just that, then attack the TPS. I can’t thank you enough, my sanity is restored!

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relton : But, cheer up. This engine was #1 in J.D. Powers survey for “Initial Engine Quality at Delivery”. In other words, it felt pretty good when you left the dealership, new.

As if I ever needed proof that JD Power was like M/T’s COTY awards, but there it is!!! Bob, when a dirt cheap 4.9L engine/wiring shows up on craigslist, it’ll get the upgrade.

+1 on the LS4. Modern engine with modern engine controls instead of mid 1980s electronics and control devices. 300+HP and FWD so it’ll fit in the chassis. The transmission is a more modern design as well.

Perfect engine swap, better than original equipment. Compared to huffing and puffing with only 135HP, the LS4 will be practically loafing.

For all of Detroit’s problems, the factory built Cadillac Fleetwood limos were special cars, even in the malaise era. The factory was very old but the workers there had pride.

Any Fleetwood limo is collectible.

My sister in law’s father bought a 1953 Fleetwood
75 limo about 35 years ago and my brother got it on the road (we ended up finding the wheel cylinders at an old auto parts store in a pretty rough part of Detroit). Huge car, maybe taller than I am at 5’6. When we took it for a test drive I didn’t want to sit on the upholstery (mohair I think) so I sat on the floor in the back. At a traffic light a then new Fleetwood pulled up next to us. This was in the late 1970s. Sitting on the floor I could see over the roof of the more modern Caddy.

It has an early air conditioning system (the blower and coil were in the trunk) and if I recall correctly the power windows were hydraulic.

It looks a lot like this funeral car only it doesn’t have a vinyl roof:

Run the car until the engine is warm. Open the hood and have someone turn off the ignition while you watch the ISC motor plunger. The plunger will reset and will click for 5 or 10 seconds after the engine stops. If the ISC motor clicks for a longer time, or if it it makes weird clicking noises while the car is running, the ISC motor is probably bad.

If you have good ignition, no vacuum leaks, clean throttle plates, EGR tubes, and a good ISC motor, I’d look at the TPS.

Before you replace the TPS, the throttle plates must be clean. With the ISC motor plunger retracted, and the throttle stop resting on the minimum idle air screw, you will need to adjust the TPS voltage to service manual specs. After the TPS is installed and adjusted, the car will need to go through an idle relearn.

The above is how I corrected the same problem you have on my Seville SLS with the 4.9, after I spend weeks replacing unnecessary parts and running down dead ends.

I would say don’t waste bandwidth looking for suggestions on how to fix a HT4100…

You were lucky that the car went 84,000 miles (which is about average for the life of a HT4100)
If it were my car I would look for a 4.5 or a 4.9 and perform an engine swap. Though this whole family of engines has issues.

Kinda sad really when you consider that GM
replaced the problematic V8-6-4 with the problematic HT4100 family and then replaced the problematic 4.1/4.5/4.9 with the problematic Head Bolt disaster that was/is the NorthStar…

Oh and while I’m wasting bandwidth…

Check that the EGR valve isn’t leaking… a leaky EGR system at idle looks exactly like a vacuum leak and will cause idle issues too.

CamaroKid, having owned a 4.9, I can tell you that they were generally not bad, although they had their quirks. The weirdest thing was the the blocks suffered from porosity problems. There was a warning tag about this in the engine bay. The tag stated that after cooling system service, stop leak goo was required to keep the coolant from leaking out of the engine! The engine block was aluminum, cast in place steel cylinder sleeves, and cast iron cylinder heads! Torque wrench use is not an option when working on these engines. This two valve pushrod engine was used by Cadillac until the mid-90’s although it looked primitive, like something out of 1954. On the plus side, the engines produced an ass load of torque at low rpm’s — my SLS would launch from stop lights.

I agree with you about the Northstar. The 4.6 never met a fluid it didn’t like to leak.

CamaroKid : Kinda sad really when you consider that GM replaced the problematic V8-6-4 with the problematic HT4100 family and then replaced the problematic 4.1/4.5/4.9 with the problematic Head Bolt disaster that was/is the NorthStar…

That might be the best sentence ever written in the Piston Slap series. Nicely done.

That said, there are plenty of HT4100s with high mileage and somewhat happy owners on the Caddy forums. Seems like annual coolant changes with that GM-rebranded Bar’s Stop Leak stuff is all you need. Or something like that. :)

This motor is in surprisingly decent condition, I suspect a little time in the right place will make it work. But if anyone has a running 4.9L Caddy for dirt cheap, email me at [email protected]

Holy heavens. I saw that one-buck eBay auction. In fact I think they had two cars (is this the one with the divider window?). I assumed it just had to be some kind of horrible scam, that the next thing would be a request for a “processing fee,” and if I paid that, I would be sent on a wild goose chase around Houston to find a nonexistent Cadillac while the scammer was cleaning out my bank account and charging high priced hookers to my credit cards.

OK, the car is real. The consensus seems to be that replacing the TPS and/or the ISC motor should make it run. But before you do that, check the oil and the coolant carefully for any sign of intermixing. Check the head gaskets for any leakage. Head gasket problems on these engines often indicate some sort of warpage problem and a new gasket might not cure it. If you have any of those problems, stop, and find a good 4.5 or 4.9 and drop it in. Never mind all the other complicated swap ideas. An early 90’s 4.9 will amaze you with the increased power.

When HT4100 cars were plentiful, I recall that a lot of them failed around 75K miles, give or take. At 84K this is still in the danger zone. If you got past 100K and cared for it, then you had a good one and you could expect another 100K. I know people who have HT4100’s with over 200K, seriously.

MadHungarian : Holy heavens. I saw that one-buck eBay auction. In fact I think they had two cars (is this the one with the divider window?).

Yes, this one has the divider. There was another one? This was the only Fleetwood 75 in their possession.

The first time I saw this auction was a good number of months ago. Late winter, early spring maybe. At that time there were two auctions — this one and a white one I think, sans divider. Maybe that one sold. I know this auction ran a few times.

MadHungarian : No kidding! We saw the black Fleetwood’s auction in February (I think) and the seller needed a full month to get a title to legally dump it on someone. I just consider ourselves lucky the Caddy didn’t make a one way trip to Pick-A-Part when it got a proper TX title.

Always glad to see another HT4100 brought back to life. I once owned an `85 Brougham (RWD) equipped with the HT4100. I picked it up for free from someone who couldn’t get it running. Turned out to be a rusted-through gas line. Fixed that, and then drove it around for awhile until the exhaust fell off and the transmission started doing its “start in the wrong gear” thing. I sold it to a guy for $400 and as far as I know he still has it. I would have repaired it and kept it, but it had a bunch of rust spots all over.